The present invention relates to fasteners for fastening aircraft structure and, more particularly, to a fastener and a related method that employs a locking feature to avoid loosening of the fastener from vibration.
A common requirement when joining panels used in building or repairing aircraft is that the fastener be resistant to the loosening effects of vibration or similar forces. In satisfying this requirement, it is necessary to set the fastener at a predetermined torque to assure enough torque for proper clamping force or preload. Too low a torque will cause inadequate fastening, and vibration applied to such a fastener will magnify the loosening effects of the vibration. Too great a torque can damage both the panel surfaces and over stress the fastener itself. Also, when joining aircraft structure, it is often necessary to use a locking feature in addition to proper torque application to further ensure that the fastener will not be loosened by vibration. This is particularly critical in areas where loose parts could cause catastrophic failure, such as in engine inlet ducts of jet aircraft.
Various types of fasteners have been developed over the years for use in building and repairing aircraft. For example, some fasteners have torque limited connections to enable setting of the fastener at a predetermined torque, while others have a locknut type arrangement to create a fastening that is more secure than standard threaded nut and bolt arrangements that incorporate cotter pins and/or lock wire to prevent disassembly.
Setting a fastener to a predetermined torque can be accomplished through the use of nuts which have torque limited connections that cooperate with the tool used to secure the nut to a bolt. For example, the fastener shown in U.S. Pat. No. 4,068,555 uses a torque limited nut which threads onto the bolt. The outer surface of the nut has external splines which mate with matching internal splines in a drive ring which fits over the nut. When a predetermined amount of torque has been applied to the drive ring, the external splines on the nut shear to prevent further application of torque. A person applying torque knows that additional torque is required if it is possible to apply more torque with the driving tool.
Fasteners having locking features such as retaining rings to prevent loosening during vibration are shown, for example, in U.S. Pat. No. 1,809,620. Similarly, U.S. Pat. No. 4,260,005 shows a fastener that uses a deformable collar to make a locking connection. However, both of these fastener arrangements require the bolt threads to engage either the retaining ring or collar. This type of locking feature is susceptible to vibrations which rotate the bolt threads back through the very path they traveled when engaging the retaining ring or collar. In effect, the threads can act to guide the bolt to disengagement within the nut. While some locking fasteners do not utilize the bolt threads to make the locking connection, such as the fastener shown in U.S. Pat. No. 1,795,570, fasteners of this type are not without their problems.
Accordingly, there has existed a definite need for a fastener for securing structural members, such as aircraft panels, which can be easily and reliably set to a predetermined torque. There also has existed a definite need for a fastener that provides a secure locking connection to prevent loosening of the fastener after it has been set. The present invention satisfies these needs and provides further related advantages.